1. Field of the Invention
The disclosure relates in general to a field device and method of operating high voltage (HV) semiconductor device applied with the same, and more particularly to a field device which effectively improves turned on threshold voltage (Vth) of parasitic field device of the HV semiconductor device.
2. Description of the Related Art
There is currently an ongoing drive toward the downscaling of device dimensions in virtually all aspects of electronic device manufacture. Smaller electronic devices tend to be more popular than larger, more bulky devices when both devices have substantially equivalent capabilities. For high voltage (HV) or Ultra high voltage (UHV) semiconductor device (such as metal-oxide-semiconductor (MOS) devices), metal lines for connecting devices in silicon processes will induce parasitic field transistor turned on when metal lines pass through specific regions. It means that when MOSFETs is under high voltage operation, maximum operating voltage is limited by turned on threshold voltage (Vth) of parasitic field devices which might be lower than the breakdown voltage of MOSFETs.
Different methods have been proposed to avoid inducing parasitic field transistor from being turned on. For example, pad is formed within a high voltage well to cause no voltage difference between drain and the field device, thereby resulting in no current passing through the field device. However, this method requires largest area for pad and has risks to cause HV-well isolation failure. Another method to avoid inducing parasitic field transistor from being turned on is increasing the thickness of oxide (ILD or IMD) on the HV-well, thereby raising the difficulty for channel reverse of HV-well under HV operation. However, this method requires longer time for growing oxide, which needs extra thermal budget, and the heat accumulation might have considerable effect on other components of the HV semiconductor device.
Accordingly, it would be one of important goals for the manufacturers to improve turned on threshold voltage (Vth) of field device without increasing any cost (e.g. thermal budget or extra mask) and device area (e.g. pad in HVN/HVP WELL), and maintain maximum operating voltage of the HV semiconductor device consequently.